1. Field of the Invention
The present invention relates to a structure for a bushing utilized at a connecting portion between a suspension member and a vehicular body of an automotive vehicle, and specifically to a slide bushing structure which permits a smooth movement of a suspension member with regard to the vehicle body, while efficiently absorbing a portion of road shock transmitted from the road wheel to the suspension member.
2. Description of the Prior Disclosure
As is generally known, various bushings are used at connecting portions of linkages utilized in automotive suspension systems. For example, a slide bushing is provided in a connecting portion between an upper control arm (upper suspension arm) of a double wishbone type independent front suspension system and a vehicle body, so as to permit a smooth pivoting movement of the upper control arm pivotably supported on the car frame by way of an inner support shaft. Such conventional bushing structures have been disclosed in Japanese Patent First Publication (Tokkai) Heisei 1-105032 and Japanese Utility Model First Publication (Jikkai) Showa 62-49033.
As shown in FIG. 1, a conventional bushing assembly 1 includes an inner cylindrical member 3 fitted onto the outer periphery of an inner support shaft 2, an outer cylindrical member 4 concentrically arranged with the inner cylindrical member 3, a cylindrical elastic member 5 disposed between the inner and outer cylindrical members 3 and 4 in close proximity to the inner periphery of the outer cylindrical member 4, and a rotatable member 6 disposed between the elastic member 5 and the inner cylindrical member 3 so that the inner periphery of the elastic member 5 is fitted onto the outer periphery of the rotatable member 6. Traditionally, the elastic member 5 is made of a rubber material, while the rotatable member 6 is made of a synthetic resin. As clearly seen in FIG. 1, a substantially cylindrical aperture 7 is defined between the inner peripheral surface 6a of the rotatable member 6 and the outer peripheral surface 3a of the inner cylindrical member 3. Such a slight aperture 7 is effective to provide a smooth rotational movement of the rotatable member 6 with respect to the inner cylindrical member 3. In order to achieve a properly high rigidity of the bushing assembly in its radial direction, the elastic member 5 is not soft but hard. The properly high rigidity of the bushing is effective to absorb input vibration acting in the radial direction of the support shaft 2. In the above noted conventional bushing structure, assuming that the inner peripheral surface of the rotatable member 6 is fitted onto the outer peripheral surface of the inner cylindrical member 3 without providing a slight aperture 7 between the two members 6 and 3 and in addition the rigidity of the bushing is high, the elastical member 5 would serve as a torsional spring having a relatively high torsional rigidity. This prevents a smooth pivoting movement of the upper control arm of the double wishbone type suspension system. For this reason, it is important to provide a proper aperture 7 between two members, namely an inner support shaft 3 and a rotatable member 6. However, when the vehicle travels on a bumpy road and the bushing receives a relatively great input vibration transmitted through the outer cylindrical member 4, the conventional bushing having the above aperture 7 experiences a great hammering noise created due to cyclical collisions occurring between the outer peripheral surface 3a of the inner cylindrical member 3 and the inner peripheral surface 6a of the rotatable member 6.